1. Field of the Invention
The present invention relates to ink-jet printers and, more particularly, is directed to an ink-jet printer of a drop-on-demand type.
2. Description of the Related Art
Conventional ink-jet printers are roughly classified into a continuous droplet type and a drop-on-demand type. Drop-on-demand type ink-jet printers are classified into an electromechanical transducer type, an electrothermal conversion type, an electrostatic adsorption type and a discharge type.
According to the continuous droplet type ink-jet printers, ink drops are exhausted from a very small nozzle, so that the thus exhausted ink drops are exhausted at a constant cycle by vibration and the ink drops are electrified with charges so that the ink drops are deflected or diffused. Although the continuous droplet type ink-jet printer becomes large in size and the ink used must be collected, the continuous droplet type ink-jet printer has a high frequency responsiveness so that it is suitable for high speed recording. Moreover, if the inner diameter of the nozzle is reduced or ink drops are divided into smaller ink droplets, then an image of high resolution can be produced.
As compared with the continuous droplet type ink-jet printer, the drop-on-demand type ink-jet printer can be simplified in structure, made compact in size, and is inexpensive as compared with the continuous droplet type one. Therefore, the drop-on-demand type ink-jet printers are widely used.
According to a print head of the drop-on-demand type ink-jet printers, the ink is jetted by a pressure caused when a piezoelectric element is deformed or by a pressure of bubbles caused when the ink is heated and boiled by a heating element.
Since the drop-on-demand type ink-jet printer is not arranged so as to constantly exhaust the ink from its nozzle unlike the continuous droplet-type ink-jet printer, it is frequently observed that the nozzle is clogged or choked-up with dried ink, denatured ink, ink dusts or the like. Hence, the ink cannot be exhausted smoothly, which unavoidably requires maintenance. Furthermore, since it is difficult for the drop-on-demand type ink-jet printer to produce a middle tone printing, it is unavoidable that this type of ink-jet printer produces the middle tone printing by utilizing a so-called dither processing in a quasi-printing fashion. Therefore, the quality of printing image is not sufficient and an image cannot be printed in full color in a full color simultaneous printing fashion. Furthermore, in this drop-on-demand type ink-jet printer, a drive mechanism thereof becomes large in size. Moreover, there is a significant restriction for modifying a nozzle in the form of a multi-nozzle. As a result, it is impossible to realize high speed printing by modifying a print head as a line-head.